This invention relates generally to a fast food service technique wherein a meal is first cooked, then refrigerated and stored and subsequently reheated without degrading the basic texture, flavor and nutritional qualities of the meal, and more particularly to a hot-air oven for reheating the food to a service temperature at which it can be dispensed to diners, the oven including a thermal two-zone arrangement which acts to accelerate the heat transfer rate and functions to raise the temperature of the food to its service level within a relatively short period.
To meet the growing need for quickly-prepared, low-cost meals, fast-food operations have been developed in which the food to be served is first cooked, then deep-freezed and stored. When an order is placed for a particular item on the menu, the selected item is withdrawn from the freezer, the frozen pre-cooked meal is then thawed and reheated.
Though fast food techniques of the type heretofore known make possible relatively inexpensive meals and expedite service, the meals provided thereby are often unappetizing. The reason for this is that while freezing is effective in preserving food and in minimizing contamination, it often does so at the expense of the quality and flavor of the product. In the course of freezing, the moisture content of the food is converted into ice crystals which act destructively; for they rupture the internal structure of the food. As a consequence, frozen food has a characteristically tasteless and mushy quality.
Moreover, in reheating a pre-cooked frozen meal it is difficult when going from the frozen state to an adequately heated condition to avoid a situation in which the core of the product is still cold even though the outer layer is quite hot. And when one seeks to ensure that the body of the food is hot throughout, there is a tendency to overheat the meal and thereby re-cook it, with a resultant loss of nutritional value and flavor.
A major factor which militates against the success of self-service fast food techniques is that the heated food is necessarily stored in a closed heat chamber which must be opened to obtain access to the product. In a mass feeding operation in which a large number of heated meals must be stored in readiness for withdrawal by diners, this involves a complicated multi-compartment structure, each with a separate door that must be opened to remove the meal and then closed.
In my above-identified copending application Ser. No. 776,772, there is disclosed a fast food service technique and apparatus therefor whereby pre-cooked food which has been refrigerated may thereafter be reheated and made directly available to customers without degrading the essential texture, flavor or nutritional qualities of the meal.
My copending application Ser. No. 776,772, discloses a hot air oven for heating tray-loaded cartridges, each constituted by a stack of sealed trays containing pre-cooked meals nested within an open carton whose side walls have holes therein to admit heated air. The oven includes a rotating turntable provided with a raised annular shelf for supporting an annular array of cartridges, the side walls of which define a hollow center core. A driven propeller is disposed within the core, the space between the shelf and the turntable forming a restricted flow passage whose inlet communicates with the core and whose outlet lies at the periphery of the turntable.
In this hot air oven, a heater assembly above the annular cartridge array produces heated air which is blown by the propeller into the hollow core. Because of the flow restriction, a substantial portion of the heated air is forced through the holes of the cartons to heat the food in the trays. The remaining portion of the heated air passes through the flow passage, the air discharged from the outlet thereof being drawn upwardly by the suction force of the propeller to create an air curtain around the cartridge array. Thus a toroidal flow pattern of heated air fully envelops the heated trays and serves to isolate the trays from the relatively cool ambient air without, however, interfering with direct access to the trays which may be withdrawn from the cartons when the food is at the desired temperature level for service to diners.
In an oven of the type disclosed in my copending application, a two-section heating assembly is provided having different wattages, whereby at the outset of heating, both sections are operative for a controllable period, hereinafter called the heat-up phase, sufficient to raise the food temperature to the desired service level, after which the main section is rendered inactive while the auxiliary section which draws much less power then serves to maintain indefinitely the heated food at the proper level, for service to diners, hereinafter called the "service phase".
During the heat-up phase, the rate of heat transfer from the hot air in the oven to the relatively cold food-loaded cartridges depends on the temperature differential; the greater the difference between the hot air temperature and the food temperature, the more rapid the rate of heat transfer.
Since the hot air temperature throughout the oven is at a fairly uniform level, the transfer rate at the outset of heating in the heat-up phase when both heater sections are operative is very rapid, but as the difference in temperature between the hot air and the food thereafter diminishes, the rate of transfer becomes increasingly slow and quite sluggish as the service temperature is approached.
Assuming that the food in the cartridges is initially at a temperature of about 40.degree. F. and it is necessary to raise the food temperature to a service level of about 150.degree. F. and further assuming a hot air temperature of about 165.degree. F., then at the outset of the heat-up phase, there will be a sharp differential giving rise to very rapid heating. But as this temperature differential diminishes in the course of the heat-up phase, the rate of heat transfer slows down. When, for example, the food temperature reaches 130.degree. F., the temperature differential relative to the heated air is only 35.degree. F., and it takes a relatively long time before the food temperature can be raised to the service temperature of 150.degree. F., at which point the heat-up phase is concluded and the service phase takes over with only one heater section operative to maintain this service temperature level.
Thus if one plots a curve of cartridge food temperature (40.degree. F. to 150.degree. F.) vs. time in the heat-up phase, the resultant curve for a hot-air temperature of 165.degree. F. will exhibit a sharp rise from 40.degree. F. to about 100.degree. F. within a fairly short time interval, the curve thereafter leveling off as the temperature goes more gradually from 100.degree. F. to 150.degree. F. As a result, the duration of the heat-up phase is unduly prolonged, which in some situations may be a practical disadvantage.
If, for instance, a fast-food installation having a hot-air oven of the type disclosed in my copending application Ser. No. 776,772 is loaded with cold food cartridges which must be made available for service to diners in about one hour after loading, this time may be inadequate to bring the food to its proper service level.
In a technique in accordance with the invention, the trays are kept under refrigeration at a temperature just above the freezing point of the pre-cooked food which, in practice, may be in a range of about 20.degree. F. to 30.degree. F.; for where the moisture content of the food is rich in dissolved salts, the freezing point may be well below 32.degree. F. It is important that the refrigeration, while close to freezing, not fall below the freezing point; for the formation of destructive ice crystals in the food must be avoided. It is also important to seal the trays to avoid the loss of moisture and volatile constituents. If, therefore, the cartridges containing the trays have just been removed from the refrigerator before being placed in the hot air oven, the necessary heat-up phase to raise the food temperature from, say, 20.degree. F. to 150.degree. F. with an oven of the type disclosed in my copending application Ser. No. 776,772 may be well over an hour, a period which is excessively long for some fast-food operations.